1. Field of the Invention
This invention concerns a process for producing cellular plastics by the polyisocyanate polyaddition process using stable emulsions containing blowing agents, and such emulsions.
2. Description of the Related Art
Production of cellular polyisocyanate polyaddition products such as cellular polyurethane elastomers and flexible, semirigid or rigid polyurethane foams by reaction of organic polyisocyanates and/or modified organic polyisocyanates with higher molecular compounds with at least two reactive hydrogens such as polyoxyalkylene polyamines and/or preferably polyhydroxyl compounds with a molecular weight of 500 to 12,000, for example, plus optional chain extenders and/or crosslinking agents with a molecular weight up to about 500 in the presence of catalysts, blowing agents, processing aids and/or additives is known from numerous patents and publications in the literature. Through a suitable choice of synthesis components, polyisocyanates, high molecular compounds with reactive hydrogens, and optional chain extenders and/or crosslinking agents, elastic or rigid cellular polyisocyanate polyaddition products as well as all the modifications inbetween can be produced by this method.
A survey of methods of producing cellular polyurethane (PU) elastomers, polyurethane (PU) foams and polyisocyanurate (PIR) foams, their mechanical properties and their use can be found, for example, in the monographs High Polymers, Volume XVI, "Polyurethanes", Parts I and II by J. H. Saunders and K. C. Frisch (Interscience Publishers, New York 1962 and 1964), Plastics Handbook, Volume VII, "Polyurethanes," 1st edition, 1966, published by R. Vieweg and A. Hochtlen and 2nd edition, 1983, published by G. Oertel (Carl Hanser Verlag, Munich) and "Integral Foams," published by H. Piechota and H. Rohr (Carl Hanser Verlag, Munich, Vienna, 1975).
Essentially two types of blowing agents are used to produce cellular plastics by the isocyanate polyaddition process: Low boiling inert liquids that evaporate under the influence of the exothermic polyaddition reaction, e.g., alkanes such as butane, pentane, etc., or preferably halogenated hydrocarbons such as methylene chloride, dichloromonofluoromethane, trichlorofluoromethane, etc., and chemical compounds that form gaseous blowing agents by means of a chemical reaction or a thermal decomposition. Examples include the reaction of water with isocyanates to form amines and carbon dioxide which takes place in synchronization with the production of polyurethane, and the cleavage of thermally unstable compounds such as azobisisobutyronitrile, which yields nitrogen as a cleavage product plus toxic tetramethylsuccinodinitrile, or azodicarbonamide, whose use as a component of a blowing agent combination is described in European Patent No. A 92,740 (CA 1,208,912). Although the latter method, whereby thermally unstable compounds such as azo compounds, hydrazides, semicarbazides, N-nitroso compounds, benzoxazines, etc. (Kunststoffe, Vol. 66, 1976, No. 10, pages 698 to 701) are usually incorporated into a prefabricated polymer or rolled onto the plastic granules and the polymer foamed by extrusion has remained of little importance industrially, the low boiling liquids with a physical action, especially chlorofluoroalkanes, are used throughout the world on a large scale to produce polyurethane foams and polyisocyanurate foams. The only disadvantage to these gaseous blowing agents is the problem of environmental pollution. When gaseous blowing agents are formed by thermal cleavage or chemical reaction, however, cleavage products and/or reactive byproducts are formed and become incorporated into the polyaddition product or are chemically bound and can lead to an unwanted change in the mechanical properties of the plastic. In the case of formation of carbon dioxide from water and diisocyanate, urea groups are formed in the polyaddition product and can lead to an improvement in compressive strength or to embrittlement of the polyurethane depending on the amount.
The mechanism of foaming in the production of polyisocyanate polyaddition products and the influence of surface active additives based on siloxane-oxalkylene copolymers on this reaction was described by B. Kanner et al. (J. of Cellular Plastics, January, 1969, pages 32 to 39). According to these and other publications, the essential requirement for forming cellular polyisocyanate polyaddition products with a uniform cell structure and good mechanical properties is a homogeneous solution of the blowing agent such as the carbon dioxide and/or inert low boiling liquids in the organic polyisocyanates and/or compounds with reactive hydrogens ("Blowing Agents for Polyurethanes," by L. M. Zwolinski in Rubber Age, July, 1975, pages 50 to 55 and British Patent No. A 904,003). If the blowing agents are not soluble in the aforementioned synthesis compounds, either large pore foams are obtained or in most cases no foams are obtained at all.
To minimize this disadvantage, U.S. Pat. No. 4,544,679 describes the use of specific polyol blends with an increased fluorocarbon solubility. An attempt has been made to obtain homogeneous solutions of the blowing agents and the polyisocyanates and/or polyols by adding sometimes substantial quantities of solubilizers (K. Tanabe, I. Kamemura and S. Kozawa, 28, SPI Conf., 1984, pages 53 to 57). Oligomeric acrylates containing polyoxyalkylene groups and fluoroalkane groups as side groups are also known, such as the oligomers containing several fluoroaliphatic side groups with 5 to 30 wt % fluorine and formulas I and II as defined below. Their synthesis and use as solubilizers in reinforced plastics by being either mixed with the plastic or applied to the fillers or reinforcing fibers is described in German Patent No. B 2,310,857 (U.S. Pat. No. 3,787,351). However, said patent contains no mention of the efficacy of the oligomeric acrylates as emulsifiers in emulsifying low boiling partially or completely fluorinated aliphatic or cycloaliphatic hydrocarbons or sulfur hexafluoride in organic polyisocyanates or higher molecular compounds that are reactive with NCO groups.
Therefore, the goal of the present invention was to replace the fluorocarbons known as blowing agents for the production of cellular plastics by the polyisocyanate polyaddition process either entirely or at least partially by using other environmentally safe blowing agents.
This problem has surprisingly been solved with the help of stable emulsions that contain a blowing agent and consist of low boiling partially or completely fluorinated (cyclo)aliphatic hydrocarbons or sulfur hexafluoride and organic optionally modified polyisocyanates or high molecular compounds that are reactive with NCO groups.